disk management, can not set drive (USB) offline

I'm trying to set up a Hyper-V Virtual machine of Windows XP on my windows 8 Pro Laptop.

My problem is getting the VM to recognize a USB drive. I found instructions that say to, on the host, use Disk Management to set the USB drive as Offline. I follow the instructions and first remove the drive letter and then right click to set if as Offline but there is no "Offline" option?

Just like this guy's problem

Anybody know why?

thanks

gj